Utilization of digital communication techniques to communicate information between a transmitting station and a receiving station has become increasingly popular in recent years. Radio communication systems, such as cellular communication systems, are exemplary of communication systems which are increasingly constructed to utilize digital communication techniques.
When a communication signal is formed utilizing a digital communication technique, an information signal is digitized and modulated upon a carrier. Because the information signal is digitized, the communication signal formed therefrom can be transmitted in discrete bursts by a transmitting station upon a communication channel. When the communication signal is received at a receiving station, the bursts are concatenated there together, thereby to recreate the communication signal.
Because communication signals can be transmitted in discrete bursts, a single carrier can be used to transmit more than one communication signal. Time slots are defined upon the carrier, and groups of time slots are formatted into frames. Channels are formed of the particular time slots defined upon the carrier. If more than one carrier is available upon which to communicate the bursts of communication signals during time slots defined upon the carriers, multiplexing, time slots are defined upon the carriers, and channels are defined to be time slot-carrier combinations.
A cellular communication system constructed pursuant to the operational standards of the global system for mobile communications (GSM) is exemplary of a communication system in which time slots are defined upon a carrier and more than one channel is defined upon a single carrier. Systems constructed pursuant to the operational standards of the United States Digital Cellular (USDC) or the Pacific Digital Cellular (PDC) standard are similarly also exemplary of a communication system in which more than one channel is defined upon a single carrier.
A radio base station of the network infrastructure of a cellular communication system includes active antenna apparatus for transducing communication signals between the base station and mobile terminals located in the vicinity of the radio base station. Conventionally, the antenna apparatus of a radio base station is formed of an omni-directional antenna or a sector antenna. A sector antenna is sometimes defined in terms of a radial area of coverage, such as a 120.degree. sector of coverage.
Antenna beam patterns formed by conventional antenna apparatus are generally static in nature. That is to say, the configurations of the antenna beam patterns generally do not vary. Omnidirectional antenna apparatus, for instance, generate omnidirectional, i.e., circular antenna beam patterns throughout which downlink transmissions, i.e., circular antenna beam patterns throughout which downlink transmissions are propagated and throughout which uplink transmissions are detected. Sector antenna apparatus are analogously operable throughout sectorized coverage areas.
Manners have been developed by which the positions of mobile terminals with which the communication signals are communicated are determined. Proposals have been set forth to utilize antenna apparatus which forms directional antenna beam patterns and make use of information of the position of a mobile terminal to select the configuration of the antenna beam pattern.
Directional antenna beam patterns can be selected to encompass the area in which a mobile terminal is positioned while excluding other areas. By forming antenna beam patterns which include elongated lobes in this manner, quality of communications in the cellular communication system can be improved.
By forming the directional antenna beam pattern, the power of a downlink transmission communicated by the radio base station to the mobile terminal is directed towards the mobile terminal. Thereby the transmission range of the radio base station is increased and the possibility that communication of the downlink transmission might interfere with other communications in the cellular communication system is reduced. Analogously, the directionality of the antenna beam pattern reduces the levels of interference of uplink transmissions received by the radio base station on an uplink channel. The sensitivity of the uplink channel is thereby also improved.
Through use of such directional antenna apparatus, the communication capacity of the cellular communication system can also be increased. The reduced levels of interference made possible by the use of such directional antenna apparatus permit such communication capacity increase. Levels of interference are generally determinative of channel reuse patterns which define the rate by which channels can be reused in a cellular communication system. Because levels of interference are reduced, a channel reuse pattern can be formed in which the channels are reused more frequently. Or, if the channel reuse pattern is left unchanged, the quality of the communication links is enhanced. Such enhancement might, for instance, increase data or speech quality of communications between the radio base station and the mobile terminal.
Capacity can also be increased by permitting more than one mobile terminal to communicate upon a single traffic channel within a single coverage area due to the directional nature of the antenna beam patterns which can be formed, communications can be effectuated with a first mobile terminal utilizing one antenna beam pattern and with a second mobile terminal utilizing a second, non-overlapping antenna beam pattern. That is to say, two or more antenna beam patterns can be formed to extend in different directions to permit communications with mobile terminals positioned in different locations within a cell. A two-fold or greater capacity increase in communication capacity over that of a conventional communication system is possible in such an arrangement.
While the proposals for utilization of antenna apparatus capable of generating directional antenna beam patterns advantageously reduces levels of interference in the communication system, cellular system standards require, in some instances, that signal energy generated by a radio base station be detectable by mobile terminals other than the mobile terminal to which communication of a communication signal is to be directed. That is to say, the standards of some cellular communication systems require that a mobile terminal be able to extract information from a downlink transmission communicated to another mobile terminal.
For instance, in some cellular communication systems, there is a requirement that energy be transmitted on a downlink carrier even if communication signals are not being communicated to an active mobile terminal. Detection of the signal energy by a mobile terminal facilitates complexity reduction and facilitates the mobile terminal to become synchronized in the communication system and to facilitate the mobile terminal to track the downlink signal.
Also, in a cellular communication system constructed according to the IS-136 standard, a mobile terminal should be able to utilize training sequences and color codes of communication signals communicated to other mobile terminals.
Such information is utilized by the mobile terminal to enhance function of equalizer circuitry of the mobile terminal. Additionally, no power control is allowed in non-active time slots if one of the slots on the carrier is active, i.e., no power control is admitted on a time slot basis in a communication system constructed according to the IS-136 standards.
In a PDC (Pacific Digital Cellular) communication system, a mobile terminal includes a two-branch, diversity antenna. Selection of an antenna branch to utilize when receiving a downlink transmission directed thereto is made responsive to measurements of signal strengths of signals transmitted to other mobile terminals. The PDC, standard specification sets forth maximum power level reductions of channels defined upon a carrier other than the channel assigned to the mobile terminal to receive downlink transmissions directed thereto.
Also, for instance, in a GSM communication system which provides for GPRS (General Packet Radio Service), a mobile terminal must also be able to detect information transmitted to other mobile terminals. Namely, an uplink state flag transmitted by a radio base station must be detectable by a mobile terminal.
To permit the requirements set forth in the operational standards of various cellular communication systems to be met, a radio base station utilizing directional antenna apparatus must be operable in a manner to permit appropriate information to be detected by mobile terminals other than mobile terminals to which downlink transmissions are to be directed.
It is in light of this background information related to antenna apparatus for transducing communication signals that the significant improvements of the present invention have evolved.